Historically, communications systems have been implemented for the purpose of providing voice and/or data communication or vehicle location information. More recently developed systems have been extensions or evolution of existing communication models. Cellular radio telephone (CRT), specialized mobile radio and mobile data radio networks, like those of IBM-Motorola and Federal Express are examples. Unfortunately, only CRT has been able to reach a significant market penetration, and CRT is orientated towards the voice communication market-place and is unsuited to economical communication of the short data-messages that typify the majority of mobile data application. The prior art is virtually void of systems which have been originally designed to provide both radio data communication and vehicle location of devices relative to a coordinate system.
Generally, the art of radio data communication and radio location is extensive. The most widely used system for position determination is presently the Loran-C system, established by the U.S. Coast Guard more than 20 years ago, for navigation on inland and coastal waters of the U.S.A. A Loran-C receiver simultaneously processes the signals received from a number of Loran-C broadcast stations, and determines its local coordinates from phase comparisons between the various signals, and from (internal) tables of the known position of the signal sources. Loran-C offers a position uncertainty of about 1/6 mile in open areas and over water. However, since the system operates at very low frequencies, having correspondingly long wavelengths, the penetration of the signals into the concrete canyons of urban areas is quite poor, and leads to unacceptable position uncertainties in those situations. Furthermore, while the Loran-C system covers the coastal regions and inland waterways of the U.S., many of the inland areas do not receive signals strong enough to provide reliable operation.
Perhaps the most significant deficiency of the Loran system is that it provides no communication path for voice or data message information.
Satellite-based systems providing a service similar to Loran-C are now in operation, such as the military and commercial versions of Global Positioning Satellite system (GPS). It can provide accurate position determination(about 50 m rms error for the commercial version of GPS) from signal received simultaneously form at least three high-orbiting satellites. In many respects, the GPS system works like a "Loran-C-in-the-sky," and performs well in open areas, but is often defeated in many consumer applications and in urban areas. This is because the power levels from the satellites are very low and the mobile transponder requires a clear, direct view of the satellites for its operation. Such a clear view is often enough not the case in urban areas to limits its use to applications only requiring transient position locating or to preclude the use of GPS-positioning in many consumer applications. Moreover, the cost of commercial GPS and Loran-C receivers is excessive and there is no means by which the location can be communicated.
Another position determining system is the "Signpost" systems. The resolution of this system is the spacing between signposts, and the position is recorded as the vehicle passes the signpost. Unfortunately, the position of the vehicle between posts is unknown.
An interesting commercial version of a signpost system, developed by Amtech Corporation, uses passive, coded tags, capable of being "scanned" by a locally strong RF signal at the signpost. Commercial versions of this system are used to keep track of the passive-responding "Toll-Tags" attached to automobiles and railroad cars. Toll-Tags have also been used for automatic toll-road fee collection.
Detailed discussion on the characteristics and error analysis of the signpost and other navigational systems is provided in "AUTOMATIC VEHICLE LOCATING SYSTEMS" by Edward E. Skomal, Van Nostrand Reinhold Company, 1981, ISBN 0-442-24495-9.
Two other vehicle locating systems worth mentioning include the North American Teletrac and the Lo-jack vehicle recovery systems. The Teletrac system uses a high frequency paging channel to activate a homing transponder on the vehicle being tracked. The transponder transmits a repeated spread-spectrum sequence, which a network of base stations receive, and from the difference in the delay between the sequence received at pairs of base stations the system determine the differences in the signal propagation flight time. From these differences a position fix can be determined by hyperbolic multilateration.
The capacity of the Teletrac system is limited to approximately 35 position fixes per second, and the system has little or no capacity to convey user information to the mobile or back to the system.
The Lo-jack system also uses a paging channel to activate a homing device installed on the stolen vehicle. Once the vehicle operator informs the recovery-service of the loss of the vehicle, the paging channel is used to activate the homing transmitter, which emits a user ID-code on its homing carrier. Police tracking vehicles, equipped with special Doppler direction finding receivers then locate the vehicle by monitoring a combination of signal-strength and direction readings of the identified carrier being received.
A number of private data-networks have recently been built to provide mobile radio data communications. Early systems have been extensions of prior voice radio systems with analog modems included in the link to provide data transmission and reception. With the advent of Specialized Mobile Radio (SMR) and the birth of Cellular Radio Telephones (which is a particular implementation of SMR), full duplex data connections became possible. These implementations, unfortunately, have been based on the current technology of voice-radio channels, which limit the capacity of the channel to practical data transmission rates in the range of 1,200 to 19,200 baud, with 2,400-4,800 baud being typical, depending on link budget and equipment cost. Several private and public radio data networks have been built along these lines by Ericsson, Motorola and others. Data-modems have recently become available from several manufacturers to operate with cellular radio telephone systems and provide typical data rates of 2,400 baud.
Some satellite-based mobile data communications have been established in the Mobile Satellite Service (MSS), and companies such a Geostar, Qualcomm and Hughes have offered the service to mainly long-distance trucking companies wishing to stay in contact with their cross-country fleets. A mobile data communication link is often combined with a Loran-C navigational system to report the approximate location of the vehicle.
The approaches used by the most recently developed location and radio data systems are mainly based on extensions of previous approaches, such as voice radio, satellite data communications and conventional navigation systems such as Loran-C or GPS. By combining these navigational systems with the radio-data systems, systems have been created that can functionally provide message delivery with radio-position determination. However, the installed cost of the equipment package for the vehicles and the on-going operating cost of message delivery have been so high that only the very largest organizations, with very high service overhead costs, have been able to economically justify the use of the available approaches. Furthermore, the maximum (saturated) message handling capacity of the systems are too low to handle many commercial or consumer applications, even if their operating cost could be lowered to the point economic feasibility. For example, a few moderate sized taxi cab fleets or a single large metropolitan police department would completely consume the vehicle-positioning capacity of a Teletrac system, and if a conventional mobile radio data system was being used to convey the message portion of the application task, less than 2,000 vehicles would saturate the system's capacity.
The problem of high-resolution position determination has been addressed in a number of different ways. Moreover, the prior art has been unable to attain a universally effective operation in an urban environment. Most mobile-radio data communication in urban environments is presently limited to moderate data rates (1200-4800 baud) by the narrow bandwidth assigned to each mobile communication channel; bandwidths typically designed for voice-grade communication. Attempts to raise the data rates above these levels have also been frustrated by the severe multipath distortion occurring in the urban mobile radio communication environment.
Accordingly, there is a need for a monolithic system for both radio-location and inexpensive message delivery capable of serving a very large subscriber base from a control center. Perhaps equally important, there is a need for such a system which provides the mobile community with an economically feasible means to implement new services and to inaugurate services that were only theoretically possible in the past.